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  Numerical modeling of the seepage erosion process around tunnels using DEM

  For a shield driven tunnel with segmental lining built in saturated soils, the tunnel convergence and differential longitudinal settlement may induce the opening and stagger of segmental joints and generate segmental cracks. Therefore, the leakage frequently happens at the locations such as the circumferential and longitudinal segmental joints, cracks and grouting holes. In the low permeability soils, the tunnel leakage provides a new drainage boundary and will cause the decrease of the pore pressure around the tunnel, thus results in the ground and tunnel settlements. The leakage induced ground and tunnel response can be significant in saturated clay and have been of a great concern in tunnel engineering (O'Reilly et al. 1991;Yi et al, 1993; Mair & Taylor, 1997; Cooper et al, 2002; Asakura and Kojima, 2003; Wongsaroj et al. 2007; Mair, 2008; Shin et al. 2002; Zhang et al. 2012). The common understanding on the effect of tunnel leakage in high permeability soil, for example sand and silty sand, is that it wouldn’t significantly reduce the pore pressure neither the ground and tunnel settlement. However, the fine particles could be easily eroded by the tunnel seepage into the tunnel through the segment joints because of the low bonding force between the particles in sand or silty sand. The seepage erosion induced hazards for dams have been widely studied (Wan & Fell, 2004; Richards, 2007; Fox, 2006; Midgley, 2013;), while it is long ignored in the tunnel engineering. The erosion of fine particles may result in the ground loss around the tunnel and increase the void ratio of soil, which will seriously reduce the strength and stiffness of the soils. With the cyclic traffic loading of subway train, further ground loss may happen. The excessive ground loss will induce the differential settlement along the tunnel, which will deteriorate the serviceability of subway tunnel. Fig 1.1 Erosion induced voids along the tunnel section

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